geom_gate: ensure readprov is null-terminated

With crafted input to the G_GATE_CMD_CREATE ioctl, geom_gate can be made
to print kernel memory to the system console, potentially revealing
sensitive data from whatever was previously in that memory page.

But but but: this is a case of the sys admin misconfiguring, and you'd
need root privileges to do this.

Submitted By: Johannes Totz <jo@bruelltuete.com>
MFC after: 2 weeks
Reviewed By: asomers
Differential Revision: https://reviews.freebsd.org/D31727